Combination Water Heaters

A combination water heater is designed to produce both domestic (potable) and space heating hot water from the same source. As shown in Figure 7-24, an inner stainless steel tank containing the domestic hot water supply is immersed inside an insulated outer tank containing the prime water used for space heating. The prime water in the outer tank is heated by a gas or oil burner. The heated prime water transfers its heat by convection to the domestic water supply stored in the inner tank. The two tanks are completely separate from one another, preventing the domestic and space heating waters from

Primary heating supply connection

Safety limit thermostat (205°F) (manual reset)

Secondary circuit (domestic water)

Steel body containing primary water

Flueway fitted with stainless steel turbulators


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Hot water E7


Cold water inlet

Baked enamel steel jacket Baffles (turbulators)

Primary circuit thermostat sensing bulb

Thermostat sensing bulb

Polyurethane insulation

Combination Water Heaters

Primary water


Heat exchanger


^"Primary heating return connection

_____ Water-cooled immersed

Combustion chamber




Primary circuit drain connection




Figure 7-24 Combination water heater.


Combination Water Heaters Combination Water Heaters

Mixing. Each combination water heater operates in conjunction with a circulating pump, expansion tank, pressure-reducing fill valve, and three-way zone valve. See Chapter 4, “Water Heaters” in Volume 3 for additional information about combination water heaters.

Steam can also be used as a source of heat in a hot-water heating system. If such is the case, then a heat exchanger (Figure 7-25) must be used to convert the steam to hot water before it reaches the heat — emitting units. The heat exchanger is commonly of the steel-shell and copper-tube design. The steam flows through the shell and the water through the tubes (Figure 7-26). The hot water returns to the steam boiler, where it is reheated and changed to steam again.

Posted in Audel HVAC Fundamentals Volume 1 Heating Systems, Furnaces, and Boilers